The present invention relates to a brake booster for automotive vehicles comprising a low-pressure casing sealingly subdivided into a low-pressure chamber and a working chamber by an axially movable wall; a reinforcement tube extending axially through the low-pressure casing having its ends fastened to the two end walls of the low-pressure casing and a rolling diaphragm in sealing abutment relative to the movable wall; a mechanically actuatable control valve for connecting the working chamber to the low-pressure chamber or to atmosphere, the control valve housing axially movable in the reinforcement tube is connected via a push rod with an actuating piston of a master cylinder, secured to the low-pressure casing on the side close to the partial vacuum, and via a sleeve encompassing the push rod is connected to radial ribs of the movable wall, the radial ribs projecting through longitudinal slots of the reinforcement tube.
When a brake booster is conventionally arranged between the automotive vehicle's splashboard and the master cylinder of the brake unit, a considerable amount of force has to be transmitted from the master cylinder to the splashboard through the brake booster casing upon actuation of the brake. To relieve the low-pressure casing from this force and to render possible a construction of the low-pressure casing of lighter weight, it is known for German Patent Application DE-OS No. 2,845,794 to arrange tensilely loaded construction elements in the form of tie rods between the two end walls of the low-pressure casing and thus between the master cylinder and the splashboard. The tie rods extend through the axially movable wall of the brake booster, and a seal is required at this point causing a relatively high constructional expenditure and a certain failure tendency.
A brake booster is described in the U.S. copending application of J. Belart and F. Weinecke, Ser. No. 061,113, filed July 26, 1979, assigned to the same asignee as the present application, in which the tie element interconnecting the two end walls of the low-pressure casing is formed by a reinforcement tube. Since the control valve housing is incorporated within the reinforcement tube and since the axially movable wall is required to be urged into a force-transmitting engagement with the control housing, connecting members have to be guided from the movable wall outside the reinforcement tube, through slots or recesses in the reinforcement tube to link with the control valve housing.
The slots or recesses are included in the area of the working chamber in the brake booster in the above-cited copending application. The axially movable wall is sealingly guided on the reinforcement tube in the area adjacent to the slots or recesses on the side close to the partial vacuum. The length of this sealing guide corresponds inevitably to the power stroke of the axially movable wall. Since the length of the slots or recesses is likewise determined by the power stroke, the length of the reinforcement tube has to be greater than double the power stroke of the movable wall resulting in a comparatively large overall length of the brake booster.
A reduced overall length of the brake booster may be accomplished by having the seal disposed between the movable wall and the reinforcement tube constructed as a rolling diaphragm abutting the reinforcement tube in a rolling manner, with the front surface of the rolling diaphragm moving axially a distance which is but half as long as the power stroke of the movable wall as disclosed, for example, in U.S. copending application of R. Becht-P. Bohm, Ser. No. 142,298, filed Apr. 12, 1908, assigned to the same assignee as the present invention. To provide an overall length of the brake booster as small as possible, the rolling diaphragm will roll out extending over the slots in the reinforcement tube.
In order to keep the thereby occasioned additional demand upon the rolling diaphragm and the thereby caused wear as small as possible, it is desired to construct the slots as narrow as possible. Accordingly, the connecting members or the ribs extending through these slots have to be constructed as narrow as possible, also. On the other hand, however, the ribs are subjected to a considerable bending stress since they transmit the total amount of boosting force of the brake booster. However, an enlargement of the ribs' dimensions in the axial direction of the brake booster is barred by the enlargement of the brake booster's overall length which would be thereby brought about.
Because of the comparatively high amount of forces required to be transmitted by the ribs extending through the slots of the reinforcement tube, it is likewise difficult to connect the ribs to the control valve housing, since the connection point of the ribs with a sleeve fitted to the control valve housing is required to project beyond the ribs as little as possible in the axial direction towards the master cylinder to keep the overall length small.